The rating of a motor should exceed that of its driven machine by a suitable margin. The selection of this margin is often made by the manufacturer of the driven machine, unless advised otherwise. The actual choice depends on various factors e.g.
• The absolute rating of either the motor or the driven machine i.e. small or large machines.
Approximate rating of the motor or machine (kW) | Margin of the motor rating above the machine rating (%) |
Up to 15 | 125 |
16.0 to 55 | 115 |
`Above 55 | 110 |
• The absolute rating of either the motor or the driven machine i.e. small or large machines.
• The function of the driven machine e.g. pump, compressor, fan, crane, conveyor.
• Expected operating level e.g. often near to maximum performance, short-term overloading permitted.
• Shape of the operating characteristic of the machine e.g. pressure (head) versus liquid flow rate in a pump.
• Change in energy conversion efficiency of the machine over its working range.
• Machine is driven at nearly constant speed.
• Machine is driven by a variable speed motor.
• Harmonic currents will be present in the motor.
• The nearest standard kW rating available of the motor.
• Ambient temperature.
Some rule-of-thumb methods are often stated in the purchasing specifications of the motor–machine unit, see for example Table 1.11, which applies to low voltage three-phase induction motors. Where the driven machine is a centrifugal type i.e. pump or compressor, the shaft power may be taken as that which occurs at the ‘end of curve’ operating point. This rule-of-thumb point is defined as being 125% of the power required at the maximum operating efficiency point on the designed curve of pressure (head) versus fluid flow rate, at the rated shaft speed. These rule-of-thumb methods can be used to check the declared performance and ratings from a machine manufacturer.
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